Compact discs (CDs) were launched as vehicles for audio. Their high data storage capabilities and low data error rates make them well suited for computer use. Consequently, the CD evolved into the CD read-only memory (CD-ROM). The high data storage capability of the CD-ROM allows it to store large amounts of text, graphics, audio, and even video.
CD audio remains a required legacy for computer CD-ROM drives, i.e., the drives must have an audio playback capability. This requirement, however, complicates the data controller of the drive. A 176,400 bytes per second data transmission frequency is required to provide proper playback of CD audio. In typical CD drives, this is the data transmission frequency from the CD.
However, frequencies as high as 2,822,400 bytes per second (16.times.176,400) are possible, and highly desirable, for non-audio (e.g. text, graphics, video). Ideally, a CD-ROM drive should support a 1.times.-16.times. frequency range for data transmission. Implementing a CD-ROM drive that provides this frequency range has to overcome many technical obstacles. Some of the obstacles are related to the analog processing of the audio data as it is received from the CD-ROM. These obstacles, if overcome, would add an unacceptable expense to mass-produced drives.
A digital video disk (DVD) can store up to eight times more information than a CD-ROM. With an appropriate DVD drive, a computer can play full-length, full-screen movies from a DVD. With the vast amount of information per unit time required for movie playback, a data transmission frequency from a DVD is greater than the continuous audio playback frequency. Thus, the lower end of the data transmission frequency range for the DVD drive may be constrained to a value many times that of the continuous audio playback frequency.
One approach is to decrease the data transmission frequency range of the drive. Nonetheless, it is desirable to provide as high a data transmission frequency as possible for quick data retrieval. Therefore, the low end of the data transmission frequency range will typically be greater than 1.times.. In such an approach, the data is provided from the CD-ROM at a frequency typically several times faster than 1.times.. However, a lower frequency (1.times.) is required for proper audio playback.
One technique to reduce the data transmission frequency to the playback frequency uses a buffer. The buffer is written with data at the data transmission frequency, and is read at the playback frequency. Since the buffer data is read at a slower frequency that the data was written, the buffer will fill up. This technique will stop providing data when the buffer is filled with data.
Once the buffer is capable of receiving more data, the drive circuitry between the CD-ROM and buffer starts writing data again. Still, a necessary requirement for audio playback is that it must be uninterrupted. Thus, the audio data read from the buffer must be the audio data immediately following the last data read. This is to prevent audio data overlaps or gaps. Otherwise, the audio playback will be interrupted and not sound "continuous." A mechanism for determining an absolute location in the audio data stream is therefore required. This requirement is met by searching the buffer for the data immediately following the last data read. In this way, the data controller circuitry reads the audio data uninterrupted.
This technique has several disadvantages. One disadvantage is searching the buffer for the data immediately following the last data read reduces the bandwidth of the data controller. Further, the searching function must be performed by either additional hardware or firmware. Another disadvantage is that the unused data written to the buffer uses valuable buffer space. Further, this unused data may be a large amount so that time is wasted searching through it. Also, the unused data amount may be so large that the sought address is not written to the buffer, and more time must be spent filling the buffer and then searching for the correct data address. These disadvantages can cause an unacceptable interrupt in the audio playback.
There remains a need for a CD-ROM drive that can provide high data transmission frequencies for non-audio data retrievals while still providing uninterruptable audio playback and yet be economical. The present invention meets this need.